Is there anyway to snap to vertices while using Edge Slide with Clamp Off?
I know this is an old question, but I've been having the same issue and managed to figure it out:


In Edit Mode, select an edge or a face that you want to use as a custom transform, this works also in object mode for objects.
Press Shift +  Numpad 7, this will align the view to selected in orthographic view.
Press Ctrl + Alt + Space bar, in order to create a custom transform.
In the Tool Shelf, that you can toggle it using T, under Create Orientation, at the bottom of the Tool Shelf, make sure you check Use View, this will align the new custom orientation to the view.
You can also rename the custom orientation to something meaningful. You can delete it or change it's name in the Properties Panel N.
Now you can use the new custom orientation instead of the default orientation, such as Global or Normal..etc. Don't use slide because it will still not react to snap, instead use transforms to move G, rotate R or scale S using the new custom transform, you can still lock axis and toggle between Global and custom transform using shortcuts.
Check Blender Documentation on Custom Orientations:
https://docs.blender.org/manual/en/dev/editors/3dview/object/editing/transform/control/orientations.html#custom-orientations